I'm at work right now, so I can't make my list too long, but here goes:
-NPCs that have life to them. Not just standing around doing nothing but shuffling their feet. I want to see NPCs conversing with each other, walking around and browsing markets, etc.
-More ambient atmosphere. For instance, in XI (please don't freak out) if you zoomed in on a bush, you just might see a grasshopper hopping about. Fish jumping out of water would be nice, too. Leaves falling from trees, etc. This all would make the game seem more "alive".
-Mini-games (that are actually fun)
Feel free to add to the list.